The Indian hedgehog-parathyroid hormone-related protein (Ihh/PTHrP) axis regulates the growth chondrocyte differentiation program (round ? flat ? prehypertrophic ? hypertrophic) that drives endochondral bone development. We have found that the Ihh-PTHrP axis is operative also in embryonic and in established articular cartilage. In the embryo, the Ihh-PTHrP axis prevents mineralization of the nascent joint. In established articular cartilage the axis is regulated by mechanical loading, with PTHrP lying upstream of Ihh in the signaling sequence. Our working hypothesis is that PTHrP and Ihh are involved in regulating the maintenance of established articular cartilage, and we propose to test this hypothesis by conditional deletion of PTHrP, Ihh signaling, or both in mice. In Specific Aim 1, we will test the hypothesis that PTHrP regulates the differentiation of articular chondrocytes and in so doing is a player in articular cartilage maintenance. We will test this idea via two sub-aims: 1) by conditionally deleting PTHrP in mid-zone articular chondrocytes via the Gdf5 promoter and b) by challenging these mice with a protocol(s) designed to highlight dysregulation of maintenance, if present. In Specific Aim 2, we propose to test the putative role of Ihh in articular cartilage maintenance by conditionally deleting Smoothened (Smo) in mid-zone articular chondrocytes via GDF5-Cre targeting. We further propose to study the combinatorial effects of the axis in articular chondrocytes by conditionally deleting PTHrP and Smo together. PUBLIC HEALTH RELEVANCE: The chondrocytes that make up articular cartilage must remain in their native, undifferentiated state in order for this cartilage to carry out its normal functions, which include lubricating the joints and cushioning load in weight-bearing joints. When chondrocytes differentiate, they begin to mineralize and degenerate, and these are hallmarks of osteoarthritis, the most common form of arthritis, and a leading cause of disability worldwide. We describe here two regulatory molecules that together seem to regulate articular chondrocyte differentiation and describe models that may provide insight into the cause of osteoarthritis at a cellular and molecular level.